1. Field of the Invention
This invention relates to radiation-sensitive linear polymeric o-nitrophenyl acetals suitable for use in the graphic arts. In particular, it pertains to radiation-sensitive poly(o-nitrophenyl acetals) formed, for example, by the linear polycondensation of o-nitrobenzaldehyde with organic diols, especially diols having a cyclic nucleus. Furthermore, the invention relates to the polyesterification of a monomer of the polyacetal, or the polyacetal itself, with, for example, an arylene dibasic acid chloride, and endcapping terminal hydroxy groups of the polyacetal by esterification with a monobasic organic acid.
2. Prior Art
Light-sensitive, positive-working polymers have been used for photoresists and on lithographic plates. Exposure of the sensitive polymer through an image-bearing intermediate element results in a solubility differential between the exposed and unexposed areas of the polymer such that treatment with an appropriate solvent washes away the exposed regions leaving the desired positive image areas for viewing on a support.
Polymers containing quinone diazide groups have drawn significant attention in the art for their good wear, printing, and developability characteristics. Such materials are disclosed, for example, in U.S. Pat. Nos. 3,759,711 (issued Sept. 18, 1973 to F. J. Rauner et al) and 3,859,099 (issued Jan. 7, 1975 to C. C. Petropoulos). Such polymers, however, are prone to thermal degradation at room temperature or higher temperatures. To prevent degradation, storage at low temperatures (0.degree.-5.degree. C.) is required.
Alternatively, it has been proposed in the art to use materials with light-sensitive o-nitroaromatic moieties appended indirectly to a polymer backbone. ("Indirectly appended", in this regard, refers to the presence of an intervening group linking the o-nitroaromatic moiety with the backbone). See, for example, U.S. Pat. No. 3,849,137. Upon exposure to actinic radiation, the nitroaromatic group is released, leaving the polymer backbone intact. While such polymers appear to be more thermally stable than quinone diazides, the presence of an intact polymer backbone after exposure unfortuantely hinders the ability of developing solutions to clean out non-image areas. Though, on one hand, the presence of a polymer backbone is undesirable after exposure, on the other, a polymer is generally necessary to provide developer and etch resistance and aid in rendering the photosensitive material coatable in the preparation of a radiation-sensitive element.
More recently, as reported by Shirai and Tanaka in the Journal of Applied Polymer Science, Volume 17, (1973), pp. 1617-1619, styrene and 2-(o-nitrophenyl)-1,3-dioxolane were cationically copolymerized to form a copolymer having o-nitrophenyl groups attached directly (i.e., without intervening linking groups) to a polymer backbone. It was theorized that such a polymer would depolymerize upon irradiation. The theory, however, is questionable in light of experimental data disclosed in the article. Furthermore, Shirai and Tanaka indicated that many attempts to increase the molecular weight of their evaluated copolymer were unsuccessful. By independent analysis, applicant has demonstrated that polymers and copolymers of the cationic polymerization type disclosed by Shirai and Tanaka are not useful in a radiation-sensitive element under normal conditions of use. In particular, when such polymers or copolymers were coated on an aluminum support, exposed to ultraviolet radiation, and subsequently treated with an aqueous alcoholic or an aqueous alcoholic alkaline developer composition, no image appeared.